Tire tread composition comprising highly reinforcing reinforced amorphous precipitated silica

ABSTRACT

Cured tread rubber compositions of tires contain cured organic rubber, from 0 to 20 phr of reinforcing carbon black, and from 40 to 120 phr of reinforcing reinforced amorphous precipitated silica wherein the silica has a surface area of from 160 to 340 m 2  /g and a pore diameter at the maximum of the volume pore size distribution function of from 5 to 19 nm. The presence of such reinforcing silica provides improved tire performance characteristics, especially improved paved highway performance, to tires having treads of the cured tread rubber compositions.

This is a continuation of application Ser. No. 08/169,797, filed Dec.20, 1993 now abandoned.

A tire is a composite of many rubbery components, each of which serves aspecific and unique function. For example, the tread is thewear-resistant component of a tire which comes into contact with theroad; tread compositions are therefore formulated for high abrasionresistance, high dry traction, high wet traction, high ice traction,good speed stability, and good casing protection. Sidewalls primarilycontrol ride and support; consequently sidewall compositions areformulated for resistance to weathering, ozone attack, abrasion, tear,and radial and circumferential cracking, and for good fatigue life.Shoulder wedges are placed under the edge of the belt to reduce interplyshear strain at the belt edge; shoulder wedge compositions are thereforeformulated for high dynamic stiffness, good adhesion, high resistance tofatigue, and high resistance to tear. The base is a rubber compositionwhich is placed between the bottom of the nonskid grooves and thecarcass; base compositions are formulated for low hysteresis, goodadhesion, fatigue and tear resistance, and high durability. The cushionis a rubber composition placed between the tread and belt or between thetread base and belt; cushion compositions are formulated to give goodadhesion, low heat buildup, good fatigue resistance, good ageresistance, and compatibility with the tread composition. The apex is arubber wedge located in the lower sidewall region above the bead; it isbonded to and encased by the plies (and chipper, if present) andprovides rigidity to the lower sidewall for bending durability andlateral stiffness. Apex compositions are formulated for good dynamicstiffness, flex fatigue, tear strength, adhesion, and durability. Theliner is a thin layer of rubber composition laminated to the inside of atubeless tire to ensure retention of compressed air; liner compositionsare formulated to provide good air and moisture impermeability, highflex-fatigue resistance, and good durability. There are, in addition tothe foregoing, several rubber compositions used to provide adhesionbetween one or more of the major rubber components and one or more ofthe mechanical components such as belts, cords, strands, and fabrics.Because the demands placed on the various rubbery components are in manyways very different, a single rubbery composition is not acceptable forall purposes in a modern pneumatic tire; rather each composition isseparately formulated to meet requirements which are in many ways quitedifferent.

Silica has been used as a reinforcing pigment, in combination withreinforcing carbon black in the tread portions of tires. See, forexample, U.S. Pat. Nos. 3,203,819; 3,451,458; 3,664,403; 3,737,334;3,746,669; 3,814,160; 3,768,537; 3,881,536; 3,884,285; 3,994,742 and5,227,425.

It has now been discovered that the presence of highly reinforcingreinforced amorphous precipitated silica, such as described in U.S. Pat.No. 5,094,829, in cured tread rubber compositions, results in improvedtire performance characteristics, especially improved paved highwayperformance, of tires having treads of such tread rubber compositions.Examples of such performance characteristics include rolling resistance,wet grip traction, and dry grip traction as described, for example, inU.S. Pat. No. 4,925,894. More specifically, it has been discovered thatthe presence of the highly reinforcing reinforced amorphous precipitatedsilica usually imparts improved cornering coefficients and often impartsimproved rolling resistance and/or ice traction. It has also beendiscovered (1) that the surface area of reinforcing reinforced amorphousprecipitated silica used in organic rubber tire tread compositions maybe somewhat lower than the 220 m² /g stated in the patent withoutunduely sacrificing the improved performance realized through use of theinvention, and (2) the value of the total intruded volume has littleeffect on highway performance. Although U.S. Pat. No. 5,094,829 doesstate (column 2, lines 19-23) that the reinforced precipitated silicastherein described may be used as reinforcing fillers forstyrene-butadiene rubber and other organic rubbers, the patent does not(1) discuss organic rubber tires, (2) distinguish between the variousdifferent organic rubber compositions of a tire, or (3) disclose anybenefit to be obtained by employing the reinforced precipitated silicain organic rubber tire treads.

Accordingly, in a tire comprising: (a) carcass having a crown; and (b)cured tread rubber composition adhered to the crown of the carcass; theinvention is the improvement wherein the cured tread rubber compositioncomprises in combination: (c) organic rubber; (d) from 0 to 20 phr ofreinforcing carbon black; and (e) from 40 to 120 phr of reinforcingreinforced amorphous precipitated silica wherein the reinforcingreinforced amorphous precipitated silica has a surface area of from 160to 340 m² /g and a pore diameter at the maximum of the volume pore sizedistribution function of from 5 to 19 nm.

The reinforcing reinforced amorphous precipitated silica can be used inthe presence of appropriate ratios of a suitable silane coupling agentas is described, for example, in U.S. Pat. Nos. 4,519,430; 4,820,751;5,227,425; and 5,162,409. However, it has also surprisingly beendiscovered that, while silane coupling agent may be present in the tiretread composition, further improved results may be obtained if the tiretread composition is substantially free of silane coupling agent.Indeed, improved results can be obtained if the tire tread compositionis substantially free of coupling agent irrespective of type. In view ofthe hydrophobic nature of the tire tread composition and the hydrophilicnature of amorphous precipitated silica, this further improvement isquite unexpected. The absence of a coupling agent is significantlyadvantageous not only because of improved performance, but also becauseof the significant savings in cost realized.

The cured tread rubber compositions used in the present invention may bespecifically formulated for use in low fuel consumption tire treads suchas is described in U.S. Pat. No. 4,748,199.

As used herein and in the claims, the carcass comprises all parts of atire except the cured tread rubber composition and, if present, one ormore intervening adhesive layers and/or a base layer. The cured treadrubber composition may be adhered to the crown of the carcass directly,that is, in the absence of one or more intervening adhesive layersand/or a base layer. Alternatively, the cured tread rubber compositionmay be adhered to the crown of the carcass via one or more interveningadhesive layers and/or a base layer. Such constructions are themselves(but not the improvement of the present invention) well known in theart.

A wide variety of organic rubbers and mixtures thereof are suitable fornormal use in the tire tread compositions employed in the invention.Examples of such organic rubbers include natural rubber;cis-1,4-polyisoprene; cis-1,4-polybutadiene; trans-1,4-polybutadiene;1,2-polybutadiene; co-(styrene-butadiene) composed of variouspercentages of styrene and the varying microstructures of polybutadienejust denoted; acrylonitrile-based rubber compositions; andisobutylene-based rubber compositions; or a mixture thereof, asdescribed in for example U.S. Pat. Nos. 4,530,959; 4,616,065; 4,748,199;4,866,131; 4,894,420; 4,925,894; 5,082,901; and 5,162,409.

The amount of organic rubber present in the cured tread rubbercomposition may vary widely. In most instances organic rubberconstitutes from 20 to 70 percent by weight of the cured tread rubbercomposition. Often organic rubber constitutes from 30 to 65 percent byweight of the cured tread rubber composition. From 37 to 60 percent byweight is preferred. The proportion of organic rubber used in preparingthe uncured tire tread composition is substantially the same as thatpresent in the cured tread rubber composition.

Any of the reinforcing carbon blacks customarily used in rubbercompositions may be used in the cured tread rubber compositions. Asingle reinforcing carbon black or a mixture of different reinforcingcarbon blacks may be used. The proportion of reinforcing carbon blackused in preparing the uncured tire tread composition is substantiallythe same as that present in the cured tread rubber composition.

The reinforcing carbon black constitutes from 0 to 20 parts per hundredparts by weight of the rubber (phr) of the cured tread rubbercomposition. Frequently, the reinforcing carbon black constitutes from 0to 15 phr of the cured tread rubber composition. Often the reinforcingcarbon black constitutes from 0 to 10 phr of the cured tread rubbercomposition.

Although both are silicas, it is important to distinguish precipitatedsilica from silica gel inasmuch as these different materials havedifferent properties. Reference in this regard is made to R. K. Iler,The Chemistry of Silica, John Wiley & Sons, New York (1979), Library ofCongress Catalog No. QD 181.S6144. Note especially pages 15-29, 172-176,218-233, 364-365, 462-465, 554-564, and 578-579, the entire disclosuresof which are incorporated herein by reference. Silica gel is usuallyproduced commercially at low pH by acidifying an aqueous solution of asoluble metal silicate, customarily sodium silicate, with acid. The acidemployed is generally a strong mineral acid such as sulfuric acid orhydrochloric acid although carbon dioxide is sometimes used. Inasmuch asthere is essentially no difference in density between the gel phase andthe surrounding liquid phase while the viscosity is low, the gel phasedoes not settle out, that is to say, it does not precipitate. Silicagel, then, may be described as a non-precipitated, coherent, rigid,three-dimensional network of contiguous particles of colloidal amorphoussilica. The state of subdivision ranges from large, solid masses tosubmicroscopic particles, and the degree of hydration from almostanhydrous silica to soft gelatinous masses containing on the order of100 parts of water per part of silica by weight, although the highlyhydrated forms are only rarely used.

Precipitated silica is usually produced commercially by combining anaqueous solution of a soluble metal silicate, ordinarily alkali metalsilicate such as sodium silicate, and an acid so that colloidalparticles will grow in weakly alkaline solution and be coagulated by thealkali metal ions of the resulting soluble alkali metal salt. Variousacids may be used, including the mineral acids and/or carbon dioxide. Inthe absence of a coagulant, silica is not precipitated from solution atany pH. The coagulant used to effect precipitation may be the solublealkali metal salt produced during formation of the colloidal silicaparticles, it may be added electrolyte such as a soluble inorganic ororganic salt, or it may be a combination of both.

Precipitated silica, then, may be described as precipitated aggregatesof ultimate particles of colloidal amorphous silica that have not at anypoint existed as macroscopic gel during the preparation. The sizes ofthe aggregates and the degree of hydration may vary widely.

Precipitated silica powders differ from silica gels that have beenpulverized in ordinarily having a more open structure, that is, a higherspecific pore volume. However, the specific surface area of precipitatedsilica as measured by the Brunauer, Emmett, Teller (BET) method usingnitrogen as the adsorbate, is often lower than that of silica gel.

Variations in the parameters and/or conditions during production resultin variations in the types of precipitated silicas produced. Althoughthey are all broadly precipitated silicas, the types of precipitatedsilicas often differ significantly in physical properties and sometimesin chemical properties. These differences in properties are importantand often result in one type being especially useful for a particularpurpose but of marginal utility for another purpose, whereas anothertype is quite useful for that other purpose but only marginally usefulfor the first purpose.

Reinforcement of precipitated silica, that is, the deposition of silicaon aggregates of previously precipitated silica, is itself known. It hasbeen found, however, that by controlling the conditions of silicaprecipitation and multiple reinforcement steps, silicas may be producedhaving properties that make them especially useful for reinforcingorganic rubbers.

Although it is not desired to be bound by any theory, it is believedthat as precipitated silica is dried, the material shrinks;consequently, pore diameters are reduced, surface area is reduced, andthe void volume is reduced. It is further believed that by sufficientlyreinforcing the silica prior to drying, a more open structure isobtained after drying. Irrespective of theory, the reinforcingreinforced amorphous precipitated silica used in the present inventionhas, on balance, large pore diameters and a large total intruded volumefor the surface area obtained. Among the reinforcing reinforcedamorphous precipitated silicas that can be used, are those described inU.S. Pat. No. 5,094,829. It has been found, however, that usingpotassium silicate as a replacement for some or all of the sodiumsilicate can result in the production of reinforcing reinforcedamorphous precipitated silica of lower surface area.

The reinforcing reinforced amorphous precipitated silicas employed inthe present invention are characterized as "reinforcing" because theyreinforce cured rubber compositions. They are characterized as"reinforced" because they are reinforced during silica preparation asdescribed above.

Reinforcing reinforced amorphous precipitated silica having, on acoating-free and impregnant-free basis, a surface area of from 160 to340 square meters per gram (m² /g) and a pore diameter at the maximum ofthe volume pore size distribution function of from 5 to 19 nanometers(nm) may be produced by a process comprising: (a) establishing aninitial aqueous alkali metal silicate solution containing from 0.5 to 4weight percent SiO₂ and having an SiO₂ :M₂ O molar ratio of from 1.6 to3.9; (b) over a period of at least 20 minutes and with agitation, addingacid to the initial aqueous alkali metal silicate solution at atemperature below 50° C. to neutralize at least 60 percent of the M₂ Opresent in the initial aqueous alkali metal solution and thereby to forma first reaction mixture; (c) over a period of from 115 to 240 minutes,with agitation, and at a temperature of from 80° C. to 95° C.,substantially simultaneously adding to the first reaction mixture: (1)additive aqueous alkali metal silicate solution, and (2) acid, therebyto form a second reaction mixture wherein the amount of the additiveaqueous alkali metal silicate solution added is such that the amount ofSiO₂ added is from 0.5 to 2 times the amount of SiO₂ present in theinitial aqueous alkali metal silicate solution established in step (a)and wherein the amount of the acid added is such that at least 60percent of the M₂ O contained in the additive aqueous alkali metalsilicate solution added during the simultaneous addition is neutralized;(d) adding acid to the second reaction mixture with agitation at atemperature of from 80° C. to 95° C. to form a third reaction mixturehaving a pH below 9; (e) aging the third reaction mixture with agitationat a pH below 9 and at a temperature of from 80° C. to 95° C. for aperiod of from 1 to 120 minutes; (f) with agitation and at a temperatureof from 80° C. to 95° C., adding to the aged third reaction mixtureadditive aqueous alkali metal silicate solution to form a fourthreaction mixture having a pH of from 7.5 to 9; (g) forming a fifthreaction mixture by adding to the fourth reaction mixture with agitationand at a temperature of from 80° C. to 95° C., a further quantity ofadditive aqueous alkali metal silicate solution and adding acid asnecessary to maintain the pH at from 7.5 to 9 during the addition of thefurther quantity of the additive aqueous alkali metal silicate solution,wherein: (1) the amount of the additive aqueous alkali metal silicatesolution added in steps (f) and (g) is such that the amount of SiO₂added in steps (f) and (g) is from 0.05 to 0.75 times the amount of SiO₂present in the third reaction mixture, and (2) the additive aqueousalkali metal silicate solution is added in steps (f) and (g) over acollective period of at least 40 minutes; (h) aging the fifth reactionmixture with agitation at a temperature of from 80° C. to 95° C. for aperiod of from 5 to 60 minutes; (i) adding acid to the aged fifthreaction mixture with agitation at a temperature of from 80° C. to 95°C. to form a sixth reaction mixture having a pH below 7; (j) aging thesixth reaction mixture with agitation at a pH below 7 and at atemperature of from 80° C. to 95° C. for a period of at least 1 minute;(k) separating reinforced precipitated silica from most of the liquid ofthe aged sixth reaction mixture; (l) washing the separated reinforcedprecipitated silica with water; and (m) drying the washed reinforcedprecipitated silica, wherein: (n) the alkali metal silicate is lithiumsilicate, sodium silicate, potassium silicate, or a mixture thereof; and(o) M is lithium, sodium, potassium, or a mixture thereof.

Optionally, prior to step (c) the first reaction mixture is aged withagitation at a temperature of from 30° C. to 95° C. for a period of from5 to 180 minutes.

The composition of the initial aqueous alkali metal silicate solutionestablished in step (a) may vary widely. Generally the initial aqueousalkali metal silicate solution comprises from 0.5 to 4 weight percentSiO₂. In many cases the initial aqueous alkali metal silicate solutioncomprises from 1 to 3 weight percent SiO₂. From 1.5 to 2.5 weightpercent SiO₂ is preferred. Usually the initial aqueous alkali metalsilicate solution has an SiO₂ :M₂ O molar ratio of from 1.6 to 3.9.Often the SiO₂ :M₂ O molar ratio is from 2.5 to 3.6. Preferably the SiO₂:M₂ O molar ratio is from 2.9 to 3.6. Frequently the SiO₂ :M₂ O molarratio is from 3.2 to 3.3.

The composition of the additive aqueous alkali metal silicate solutionmay also vary widely. Usually the additive aqueous alkali metal silicatesolution comprises from 2 to 30 percent by weight SiO₂. Often theadditive aqueous alkali metal silicate solution comprises from 10 to 15percent by weight SiO₂. From 12 to 13 weight percent SiO₂ is preferred.Frequently the additive aqueous alkali metal silicate solution has anSiO₂ :M₂ O molar ratio of from 1.6 to 3.9. In many cases the SiO₂ :M₂ Omolar ratio is from 2.5 to about 3.6. Preferably the SiO₂ :M₂ O molarratio is from 2.9 to 3.6. Often the SiO₂ :M₂ O molar ratio is from 3.2to 3.3. Additive aqueous alkali metal silicate solution having the samecomposition may be used throughout the various silicate additions, oradditive aqueous alkali metal silicate solutions having differingcompositions may be used in different silicate addition steps.

The acid used in the process may also vary widely. In general, the acidadded in steps (b), (c), and (g) should be strong enough to neutralizealkali metal silicate and cause precipitation of silica. The acid addedin steps (d) and (i) should be strong enough to reduce the pH to desiredvalues within the specified ranges. The acid used in the various acidaddition steps may be the same or different, but preferably it is thesame. A weak acid such as carbonic acid produced by the introduction ofcarbon dioxide to the reaction mixture may be used for precipitation ofsilica, but a stronger acid must be used in steps (d) and (i) when it isdesired to reduce the pH to values below 7. It is preferred to usestrong acid throughout the process. Examples of the strong acids includesulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, andacetic acid. The strong mineral acids such as sulfuric acid,hydrochloric acid, nitric acid, and phosphoric acid are preferred;sulfuric acid is especially preferred.

The acid addition of step (b) is made over a period of at least 20minutes. Frequently the acid addition of step (b) is made over a periodof from 20 to 60 minutes. From 26 to 32 minutes is preferred.

The temperature of the reaction mixture during the acid addition of step(b) is below 50° C. From 30° C. to 40° C. is preferred.

At least 60 percent of the M₂ O present in the initial aqueous alkalimetal silicate solution is neutralized during the acid addition of step(b). As much as 100 percent of the M₂ O may be neutralized if desired.Preferably from 75 to 85 percent of the M₂ O is neutralized.

The additions made in step (c) are made over a period of from 115 to 240minutes. Preferably the additions are made over a period of from 115 to125 minutes.

The temperature of the reaction mixture during the additions of step (c)is from 80° C. to 95° C. From 90° C. to 95° C. is preferred.

In step (c), the amount of additive aqueous alkali metal silicate addedis such that the amount of SiO₂ added is from 0.5 to 2 times the amountof SiO₂ present in the initial aqueous alkali metal silicate solutionestablished in step (a). From 0.9 to 1.1 times the SiO₂ present in theinitial aqueous alkali metal silicate solution is preferred.

The amount of acid added in step (c) is such that at least 60 percent ofthe M₂ O contained in the additive aqueous alkali metal silicatesolution added in step (c) is neutralized. As much as 100 percent ofsuch M₂ O may be neutralized if desired. Preferably from 75 to 85percent of the M₂ O is neutralized.

The temperature of the reaction mixture during the acid addition of step(d) is from 80° C. to 95° C. From 90° C. to 95° C. is preferred.

In step (d), the acid is added such that the pH of the third reactionmixture is below 9. Often the pH is from 2.5 to below 9. A pH of from 4to 8.9 is preferred.

Similarly, the third reaction mixture is aged in step (e) at a pH below9. Often the pH is from 2.5 to below 9. A pH of from 4 to 8.9 ispreferred.

The temperature of the third reaction mixture during the aging of step(e) is from 80° C. to 95° C. From 90° C. to 95° C. is preferred.

The aging in step (e) is for a period of from 1 to 120 minutes. In manycases the third reaction mixture is aged for a period of from 15 to 120minutes. A period of from 15 to 30 minutes is preferred.

The temperature of the reaction mixture during the addition of additiveaqueous alkali metal silicate solution in step (f) is from 80° C. to 95°C. From 90° C. to 95° C. is preferred.

The pH of the fourth reaction mixture formed in step (f) is from 7.5 to9. A pH of from 8 to 9 is preferred.

Acid is added in step (g) as necessary to maintain the pH of thereaction mixture at from 7.5 to 9 during the addition of the furtherquantity of additive aqueous alkali metal silicate solution. A pH offrom 8 to 9 is preferred.

The amount of additive aqueous alkali metal silicate solution added insteps (f) and (g) is such that the amount of SiO₂ added in steps (f) and(g) is from 0.05 to 0.75 times the amount of SiO₂ present in the thirdreaction mixture. Preferably the amount of additive aqueous alkali metalsilicate solution added in steps (f) and (g) is such that the amount ofSiO₂ added in steps (f) and (g) is from 0.25 to 0.6 times the amount ofSiO₂ present in the third reaction mixture.

The additive alkali metal silicate solution is added in steps (f) and(g) over a collective period of at least 40 minutes. A collective periodof from 40 to 240 minutes is often employed. A collective period of from70 to 100 minutes is preferred.

The temperature of the fifth reaction mixture during the aging of step(h) is from 80° C. to 95° C. From 90° C. to 95° C. is preferred.

In step (h), the fifth reaction mixture is aged for a period of from 5to 60 minutes. A period of from 30 to 60 minutes is preferred.

The temperature of the reaction mixture during the acid addition of step(i) is from 80° C. to 95° C. From 90° C. to 95° C. is preferred.

In step (i), the acid is added such that the pH of the sixth reactionmixture is below 7. Often the pH is from 2.5 to below 7. A pH of from 4to 5 is preferred.

The sixth reaction mixture is aged in step (j) at a pH below 7. In manycases the pH is from 2.5 to below 7. A pH of from 4 to 5 is preferred.

The temperature of the sixth reaction mixture during the aging of step(j) is from 80° C. to 95° C. From 90° C. to 95° C. is preferred.

In step (j), the sixth reaction mixture is aged for a period of at least1 minute. Often the aging period is at least 30 minutes. An aging periodof at least 50 minutes is preferred.

The separation of step (k) may be accomplished by one or more techniquesfor separating solids from liquid such as, for example, filtration,centrifugation, decantation, and the like.

The washing of step (l) may be accomplished by any of the proceduresknown to the art for washing solids. Examples of such procedures includepassing water through a filter cake, and reslurring the reinforcedprecipitated silica in water followed by separating the solids from theliquid. One washing cycle or a succession of washing cycles may beemployed as desired. The primary purpose of washing is to remove saltformed by the various neutralizations to desirably low levels. Usuallythe reinforced precipitated silica is washed until the concentration ofsalt in the dried reinforced precipitated silica is less than or equalto 2 percent by weight. Preferably the reinforced precipitated silica iswashed until the concentration of salt is less than or equal to 0.7percent by weight.

The drying of step (m) may also be accomplished by one or more knowntechniques. For example, the reinforced precipitated silica may be driedin an air oven or in a vacuum oven. Preferably the reinforcedprecipitated silica is dispersed in water and spray dried in a column ofhot air. The temperature at which drying is accomplished is notcritical, but the usual practice is to employ temperatures of at least70° C. Generally the drying temperature is less than 700° C. In mostcases drying is continued until the reinforced precipitated silica hasthe characteristics of a powder. Ordinarily the dried reinforcedprecipitated silica is not absolutely anhydrous but contains bound water(from 2 to 5 weight percent) and adsorbed water (from 1 to 7 weightpercent) in varying amounts, the latter depending partly upon theprevailing relative humidity. Adsorbed water is that water which isremoved from the silica by heating at 105° C. for 24 hours atatmospheric pressure in a laboratory oven. Bound water is that waterwhich is removed by additionally heating the silica at calcinationtemperatures, for example, from 1000° C. to 1200° C.

The degrees of agitation used in the various steps of the invention mayvary considerably. The agitation employed during the addition of one ormore reactants should be at least sufficient to provide a thoroughdispersion of the reactants and reaction mixture so as to avoid morethan trivial locally high concentrations of reactants and to ensure thatsilica deposition occurs substantially uniformly thereby avoidinggellation on the macro scale. The agitation employed during aging shouldbe at least sufficient to avoid settling of solids to ensure that silicadeposition occurs substantially uniformly throughout the mass of silicaparticles rather than preferentially on those particles at or near thetop of a settled layer of particles. The degrees of agitation may, andpreferably are, greater than these minimums. In general, vigorousagitation is preferred.

A preferred embodiment of a process for producing reinforced amorphousprecipitated silica having, on a coating-free and impregnant-free basis,a surface area of from 160 to 340 m² /g and a pore diameter at themaximum of the volume pore size distribution function of from 5 to 19nm, is the process comprising: (a) establishing an initial aqueousalkali metal silicate solution containing from 0.5 to 4 weight percentSiO₂ and having an SiO₂ :M₂ O molar ratio of from 1.6 to 3.9; (b) over aperiod of at least 20 minutes and with agitation, adding acid to theinitial aqueous alkali metal silicate solution at a temperature of from30° C. to 40° C. to neutralize from 75 to 85 percent of the M₂ O presentin the initial aqueous alkali metal solution and to form a firstreaction mixture; (c) over a period of from 115 to 125 minutes, withagitation, and at a temperature of from 90° C. to 95° C., substantiallysimultaneously adding to the first reaction mixture: (1) additiveaqueous alkali metal silicate solution, and (2) acid, to form a secondreaction mixture wherein the amount of the additive aqueous alkali metalsilicate solution added is such that the amount of SiO₂ added is from0.9 to 1.1 times the amount of SiO₂ present in the initial aqueousalkali metal silicate solution established in step (a) and wherein theamount of the acid added is such that from 75 to 85 percent of the M₂ Ocontained in the additive aqueous alkali metal silicate solution addedduring the simultaneous addition is neutralized; (d) adding acid to thesecond reaction mixture with agitation at a temperature of from 90° C.to 95° C. to form a third reaction mixture having a pH of from 4 to 9;(e) aging the third reaction mixture with agitation at a temperature offrom 90° C. to 95° C. for a period of from 15 to 30 minutes; (f) withagitation and at a temperature of from 90° C. to 95° C., adding to theaged third reaction mixture additive aqueous alkali metal silicatesolution to form a fourth reaction mixture having a pH of from 8 to 9;(g) forming a fifth reaction mixture by adding to the fourth reactionmixture with agitation and at a temperature of from 90° C. to 95° C., afurther quantity of additive aqueous alkali metal silicate solution andadding acid as necessary to maintain the pH at from 8 to 9 during theaddition of the further quantity of the additive aqueous alkali metalsilicate solution, wherein: (1) the amount of the additive aqueousalkali metal silicate solution added in steps (f) and (g) is such thatthe amount of SiO₂ added in steps (f) and (g) is from 0.25 to 0.6 timesthe amount of SiO₂ present in the third reaction mixture, and (2) theadditive aqueous alkali metal silicate solution is added in steps (f)and (g) over a collective period of from 70 to 100 minutes; (h) agingthe fifth reaction mixture with agitation at a temperature of from 90°C. to 95° C. for a period of from 30 to 60 minutes; (i) adding acid tothe aged fifth reaction mixture with agitation at a temperature of from90° C. to 95° C. to form a sixth reaction mixture having a pH of from 4to 5; (j) aging the sixth reaction mixture with agitation at atemperature of from 90° C. to 95° C. for a period of at least 50minutes; (k) separating reinforced precipitated silica from most of theliquid of the aged sixth reaction mixture; (l) washing the separatedreinforced precipitated silica with water; and (m) drying the washedreinforced precipitated silica, wherein: (n) the alkali metal silicateis lithium silicate, sodium silicate, potassium silicate, or a mixturethereof; and (o) M is lithium, sodium, potassium, or a mixture thereof.

It is understood that one or more ranges in the preferred embodiment maybe used in lieu of the corresponding broader range or ranges in thebroader description of the process.

As used in the present specification and claims, the surface area of thereinforcing reinforced amorphous precipitated silica is the surface areadetermined by the Brunauer, Emmett, Teller (BET) method according toASTM C 819-77 using nitrogen as the adsorbate but modified by outgassingthe system and the sample for one hour at 180° C. The surface area isfrom 160 to 340 m² /g. In many cases the surface area is from 180 to 340m² /g. From 200 to 340 m² /g is preferred.

The volume average pore size distribution function of the reinforcingreinforced amorphous precipitated silica is determined by mercuryporosimetry using an Autoscan mercury porosimeter (Quantachrome Corp.)in accordance with the accompanying operating manual. In operating theporosimeter, a scan is made in the high pressure range (from 103kilopascals absolute to 227 megapascals absolute). The volume pore sizedistribution function is given by the following equation: ##EQU1##where: D_(v) (d) is the volume pore size distribution function, usuallyexpressed in cm³ /(mm.g);

d is the pore diameter, usually expressed in nm;

P is the pressure, usually expressed in pounds per square inch,absolute; and

V is the pore volume per unit mass, usually expressed in cm³ /g.

Dv(d) is determined by taking ΔV/ΔP for small values of ΔP from either aplot of V versus P or preferably from the raw data. Each value of ΔV/ΔPis multiplied by the pressure at the upper end of the interval anddivided by the corresponding pore diameter. The resulting value isplotted versus the pore diameter. The value of the pore diameter at themaximum of the volume pore size distribution function is then taken fromthe plotted graph. Numerical procedures may be used rather thangraphical when desired. For the reinforcing reinforced amorphousprecipitated silica used in the present invention the pore diameter atthe maximum of the volume pore size distribution function is from 5 to19 nm. Preferably the pore diameter at the maximum of the function isfrom 8 to 18 nm.

The ultimate particle size of the reinforcing reinforced amorphousprecipitated silica may be widely varied. Usually the average ultimateparticle size (irrespective of whether or not the ultimate particles areagglomerated) is less than 0.1 micrometer as determined by transmissionelectron microscopy. Often the average ultimate particle size is lessthan 0.05 micrometer. Preferably the average ultimate particle size ofthe reinforcing reinforced amorphous precipitated silica is less than0.03 micrometer.

The reinforcing reinforced amorphous precipitated silica used in thepresent invention is particulate. As present in the cured tread rubbercomposition, the reinforcing reinforced amorphous precipitated silicaparticles may be in the form of ultimate particles, aggregates ofultimate particles or a combination of both. In most cases, at least 90percent by weight of the reinforcing reinforced amorphous precipitatedsilica used in preparing the cured tread rubber composition has grossparticle sizes in the range of from 1 to 40 micrometers as determined byuse of a Model TAII Coulter Counter® (Coulter Electronics, Inc.)according to ASTM C 690-80 but modified by stirring the filler for 10minutes in Isoton® II electrolyte (Curtin Matheson Scientific, Inc.)using a four-blade, 4.445 centimeter diameter propeller stirrer.Preferably at least 90 percent by weight of the reinforcing reinforcedamorphous precipitated silica has gross particle sizes in the range offrom 5 to 30 micrometers. It is expected that the sizes of filleragglomerates may be reduced during processing of the ingredients toprepare the cured tread rubber composition. Accordingly, thedistribution of gross particle sizes in the cured tread rubbercomposition may be smaller than in the raw reinforcing reinforcedamorphous precipitated silica itself.

The neutralization of alkali metal silicate with acid to produce thereinforcing reinforced amorphous precipitated silica used in theinvention also produces alkali metal salt of the acid(s) used forneutralization as by-product. It is preferred that the amount of suchsalt associated with the reinforcing reinforced amorphous precipitatedamorphous precipitated silica product be low. When the reinforcedamorphous precipitated silica is separated from the liquid of the agedsixth reaction mixture, most of the salt is removed with the liquid.Further amounts of salt may conveniently be removed by washing theseparated reinforced amorphous precipitated silica with water. Ingeneral, the greater the amount of water used for washing, the lowerwill be the salt content of the final dried product. It is usuallypreferred that reinforcing reinforced amorphous precipitated silicacontain less than 1 percent by weight alkali metal salt. It is oftenparticularly preferred that the reinforcing reinforced amorphousprecipitated silica contain less than 0.7 percent by weight alkali metalsalt.

The reinforcing reinforced amorphous precipitated silica constitutesfrom 40 to 120 phr of the cured tread rubber composition. Often thereinforcing reinforced amorphous precipitated silica constitutes from 40to 100 phr of the cured tread rubber composition. Preferably thereinforcing reinforced amorphous precipitated silica constitutes from 40to 80 phr of the cured tread rubber composition.

There are many other materials which are customarily and/or optionallypresent in the cured tread rubber composition. These include suchmaterials as vulcanizing agent (usually, but not necessarily, sulfur),accelerator, lubricant, wax, antioxidant, semi-reinforcing carbon black,non-reinforcing carbon black, other pigment, stearic acid, and/or zincoxide. The listing of such materials is by no means exhaustive. Theseand other ingredients may be employed in their customary amounts fortheir customary purposes so long as they do not seriously interfere withgood rubber tire tread formulating practice.

The curable tire tread composition may be formed from the ingredients inany manner known to the art. Mixing and milling are most commonly used.Similarly, a tire may be built, molded, and cured using the curable tiretread composition according to any of the general methods and techniquesknown to the art. In the region where the sidewall and tread are joined,the structure is usually tread over sidewall (TOS) or sidewall overtread (SWOT). The TOS structure is characterized by a peripheral insideface of the tread rubber being adhered to the outside face of the upperend portion of the sidewall. The SWOT structure is characterized by theinside face of the upper end portion of the sidewall being in closeconnection with a side face of the tread rubber. See U.S. Pat. No.5,088,537. The TOS structure, the SWOT structure, or other structuresmay be used, but because of the greater ease with which retreading canbe accomplished, the TOS structure is preferred.

The invention is further described in conjunction with the followingexamples which are to be considered illustrative rather than limiting,and in which all parts are parts by weight and all percentages arepercentages by weight unless otherwise specified.

EXAMPLE A

An initial aqueous sodium silicate solution in the amount of 58.881liters was established in a reactor. The initial aqueous sodium silicatesolution contained about 2 weight percent SiO₂ and had an SiO₂ :Na₂ Omolar ratio of about 3.3. The initial aqueous sodium silicate solutionwas heated to 34° C. and over a period of 28 minutes and with agitation,26.708 liters of about 2 weight percent aqueous sulfuric acid was addedto the initial aqueous alkali metal silicate solution thereby toneutralize about 80 percent of the Na₂ O and to form a first reactionmixture. Over a period of 121 minutes, with agitation, and at atemperature of 80° C., a stream of 9.059 liters of additive aqueoussodium silicate solution containing about 13 weight percent SiO₂ andhaving an SiO₂ :Na₂ O molar ratio of about 3.3, and a stream of 15.321liters of about 4 weight percent aqueous sulfuric acid were addedsimultaneously to the first reaction mixture to form a second reactionmixture. The pH of the second reaction mixture was 9.1. A stream ofabout 8 liters of about 4 weight percent aqueous sulfuric acid was addedto the second reaction mixture with agitation at a temperature of 80° C.to form a third reaction mixture having a pH of 4.5. The third reactionmixture was aged with agitation at 80° C. for 30 minutes. The aged thirdreaction mixture was split into two approximately equal portions. Withagitation, 0.45 liter of additive aqueous sodium silicate solutioncontaining about 13 weight percent SiO₂ and having an SiO₂ :Na₂ O molarratio of about 3.3 was added to one portion of the aged third reactionmixture at 80° C. to form a fourth reaction mixture having a pH of 8.5.A fifth reaction mixture was formed by adding to the fourth reactionmixture with agitation and at a temperature of 80° C., 2.271 liters ofadditive aqueous sodium silicate solution containing about 13 weightpercent SiO₂ and having an SiO₂ :Na₂ O molar ratio of about 3.3 and byadding 5.5 liters of about 4 weight percent aqueous sulfuric acidsimultaneously to maintain the pH at about 8.5. The sequential additionsto form the fourth and fifth reaction mixtures were made over acollective time period of 43 minutes. The fifth reaction mixture wasaged with agitation at 80° C. for 45 minutes. With agitation, 1.5 litersof about 4 weight percent aqueous sulfuric acid was added to the agedfifth reaction mixture to form a sixth reaction mixture having a pH of4.5. The sixth reaction mixture was aged with agitation at 80° C. for 60minutes. The aged sixth reaction mixture was vacuum filtered using aseries of Buchner funnels. Just before air could be pulled through eachfilter cake, the addition of 16 liters of water to the funnel was begunfor the purpose of washing the filter cake. Air was briefly pulledthrough the washed filter cake. The wet filter cake contained 9.9percent solids by weight. After being removed from the funnels, the wetfilter cakes were stirred with a propeller type agitator to form a solidin liquid suspension. The suspension was dried in a Niro spray drier(inlet temperature about 360° C.; outlet temperature about 128° C.) toform a batch of dried reinforced precipitated silica. The product had asurface area of 333 m² /g, a pore diameter at the maximum of the volumepore size distribution function of 9 nm, and a total intruded volume of3.21 cm³ /g. The product was micronized in a fluid energy mill usingcompressed air as the working fluid.

EXAMPLE B

An initial aqueous sodium silicate solution in the amount of 340.7liters was established in a reactor. The initial aqueous sodium silicatesolution contained about 2 weight percent SiO₂ and had an SiO₂ :Na₂ Omolar ratio of about 3.3. The initial aqueous sodium silicate solutionwas heated to 37° C. and over a period of 30 minutes and with agitation,2.449 liters of about 30 weight percent aqueous sulfuric acid and137.426 liters of water were added as separate streams to the initialaqueous alkali metal silicate solution to neutralize about 80 percent ofthe Na₂ O and to form a first reaction mixture. The first reactionmixture was heated with agitation to 95° C. During the heat-up 74.8liters of water was added. The diluted first reaction mixture was thenaged with agitation at 95° C. for 60 minutes. Over a period of 120minutes, with agitation, and at a temperature of 95° C., a stream of52.41 liters of additive aqueous sodium silicate solution containingabout 13 weight percent SiO₂ and having an SiO₂ :Na₂ O molar ratio ofabout 3.3 and a stream of 9.449 liters of about 30 weight percentaqueous sulfuric acid were added to the aged diluted first reactionmixture to form a second reaction mixture. The pH of the second reactionmixture was 9.1. A stream of about 8 liters of about 30 weight percentaqueous sulfuric acid was added to the second reaction mixture withagitation at a temperature of 95° C. to form a third reaction mixturehaving a pH of 4.5. The third reaction mixture was aged with agitationat 95° C. for 30 minutes. With agitation, 6.57 liters of additiveaqueous sodium silicate solution containing about 13 weight percent SiO₂and having an SiO₂ :Na₂ O molar ratio of about 3.3 was added to the agedthird reaction mixture at 95° C. to form a fourth reaction mixturehaving a pH of 8.7. A fifth reaction mixture was formed by adding to thefourth reaction mixture with agitation and at a temperature of 95° C.,30.18 liters of additive aqueous sodium silicate solution containingabout 13 weight percent SiO₂ and having an SiO₂ :Na₂ O molar ratio ofabout 3.3, and by adding 6 liters of about 30 weight percent aqueoussulfuric acid as necessary to maintain the pH at about 8.7. Thesequential additions to form the fourth and fifth reaction mixtures weremade over a collective time period of 84 minutes. The fifth reactionmixture was aged with agitation at 95° C. for 45 minutes. Withagitation, 3.5 liters of about 30 weight percent aqueous sulfuric acidwas added to the aged fifth reaction mixture to form a sixth reactionmixture having a pH of 4.5. The sixth reaction mixture was aged withagitation for 60 minutes maintaining 95° C. and thereafter for about 900minutes without temperature maintenance. The temperature at theconclusion of the 900 minute period was 66° C. The aged sixth reactionmixture was filtered in a filter press. The filter cake was washed withwater until the conductivity of the filtrate had dropped to 90micromhos/cm. The wet filter cake and added water were mixed with aCowles blade to form a solid in liquid suspension containing 9.7 percentsolids by weight. The suspension was dried in a Niro spray drier (inlettemperature about 360° C.; outlet temperature about 128° C.) to form thereinforced precipitated silica product. The product had a surface areaof 232 m² /g, a pore diameter at the maximum of the volume pore sizedistribution function of 14 nm, and a total intruded volume of 3.09 cm³/g. The product was micronized in a fluid energy mill using compressedair as the working fluid.

EXAMPLE C

An initial aqueous sodium silicate solution in the amount of 41314liters was established in a reactor. The initial aqueous sodium silicatesolution contained about 2 weight percent SiO₂ and had an SiO₂ :Na₂ Omolar ratio of about 3.2. The initial aqueous sodium silicate solutionwas heated to 34° C. and over a period of 33 minutes and with agitation,1086 liters of about 30 weight percent aqueous sulfuric acid and 11356liters of water were added to the initial aqueous alkali metal silicatesolution to neutralize about 80 percent of the Na₂ O and to form a firstreaction mixture. The first reaction mixture was heated with agitationto 95° C. over a period of about 2 hours. The first reaction mixture wasthen aged with agitation at 95° C. for 65 minutes. A total of 2557liters of water were added during the heating and aging periods. Over aperiod of 119 minutes, with agitation, and at a temperature of 95° C., astream of 6314 liters of additive aqueous sodium silicate solutioncontaining about 12.6 weight percent SiO₂ and having an SiO₂ :Na₂ Omolar ratio of about 3.2, a stream of 1124 liters of about 30 weightpercent aqueous sulfuric acid, and a stream of 549 liters of water wereadded simultaneously to the first reaction mixture to form a secondreaction mixture. The pH of the second reaction mixture was 9.6. Astream of about 777 liters of about 30 weight percent aqueous sulfuricacid and a stream of 117 liters of water were added to the secondreaction mixture with agitation at a temperature of 95° C. to form athird reaction mixture having a pH of 4.5. The third reaction mixturewas aged with agitation at 95° C. for 30 minutes during which period 46liters of water was added. With agitation, water and 890 liters ofadditive aqueous sodium silicate solution containing about 12.6 weightpercent SiO₂ and having an SiO₂ :Na₂ O molar ratio of about 3.2 wasadded to the aged third reaction mixture at 95° C. to form a fourthreaction mixture having a pH of 8.5. A fifth reaction mixture was formedby adding to the fourth reaction mixture with agitation and at atemperature of 95° C., water and 3528 liters of additive aqueous sodiumsilicate solution containing about 12.6 weight percent SiO₂ and havingan SiO₂ :Na₂ O molar ratio of about 3.2 and by adding 846 liters ofabout 30 weight percent aqueous sulfuric acid as necessary to maintainthe pH at about 8.5. The sequential additions to form the fourth andfifth reaction mixtures were made over a collective time period of 80minutes. The fifth reaction mixture was aged with agitation at 95° C.for 45 minutes. With agitation, water and 259 liters of about 30 weightpercent aqueous sulfuric acid were added to the aged fifth reactionmixture to form a sixth reaction mixture having a pH of 4.5. A total of568 liters of water was added during formation of the fourth through thesixth reaction mixtures. The sixth reaction mixture was aged withagitation and without temperature maintenance for 653 minutes. The finaltemperature was 82° C. The aged sixth reaction mixture was divided intotwo batches of about 40504 liters and 39747 liters, respectively. Eachbatch was filtered in a filter press. The filter cakes were washed withwater until the conductivity of the filtrate had dropped to about 5micromhos/cm. A portion of the washed filter cakes from the first filterpress batch was removed and set aside. The remainder of the washedfilter cakes and added water were mixed with a Cowles blade to form asolid in liquid suspension containing 12 percent solids by weight. Thesuspension was dried in a Bowen spray drier (inlet temperature about620° C.; outlet temperature about 130° C.) to form the reinforcedprecipitated silica product. The product had a surface area of 236 m²/g, a pore diameter at the maximum of the volume pore size distributionfunction of 15 nm, and a total intruded volume of 3.2 cm³ /g.

EXAMPLE D

An initial aqueous potassium silicate solution in the amount of 64.30liters was established in a reactor. The initial aqueous potassiumsilicate solution contained about 2 weight percent SiO₂ and had an SiO₂:K₂ O molar ratio of about 3.05. The initial aqueous potassium silicatesolution was heated to 38° C. and over a period of 29 minutes and withagitation, 27.20 liters of about 2 weight percent aqueous sulfuric acidwas added to the initial aqueous alkali metal silicate solution therebyto neutralize about 80 percent of the K₂ O and to form a first reactionmixture. Over a period of 120 minutes, with agitation, and at atemperature of 94° C., a stream of 10.050 liters of additive aqueouspotassium silicate solution containing about 13 weight percent SiO₂ andhaving an SiO₂ :K₂ O molar ratio of about 3.05, and a stream of 15.60liters of about 4 weight percent aqueous sulfuric acid were addedsimultaneously to the first reaction mixture to form a second reactionmixture. The pH of the second reaction mixture was 9.4. A stream ofabout 8 liters of about 4 weight percent aqueous sulfuric acid was addedto the second reaction mixture with agitation at a temperature of 95° C.to form a third reaction mixture having a pH of 4.5. The third reactionmixture was aged with agitation at 95° C. for 30 minutes. Withagitation, 1.1 liters of additive aqueous potassium silicate solutioncontaining about 13 weight percent SiO₂ and having an SiO₂ :K₂ O molarratio of about 3.05 was added to the aged third reaction mixture at 94°C. to form a fourth reaction mixture having a pH of 8.5. A fifthreaction mixture was formed by adding to the fourth reaction mixturewith agitation and at a temperature of 94° C., 6.933 liters of additiveaqueous potassium silicate solution containing about 13 weight percentSiO₂ and having an SiO₂ :K₂ O molar ratio of about 3.05 and by adding9.5 liters of about 4 weight percent aqueous sulfuric acidsimultaneously to maintain the pH at about 8.5. The sequential additionsto form the fourth and fifth reaction mixtures were made over acollective time period of 83 minutes. The fifth reaction mixture wasaged with agitation at 94° C. for 45 minutes. With agitation, 3.0 litersof about 4 weight percent aqueous sulfuric acid was added to the agedfifth reaction mixture to form a sixth reaction mixture having a pH of4.5. The sixth reaction mixture was aged with agitation at 94° C. for 60minutes. The aged sixth reaction mixture was vacuum filtered using aseries of Buchner funnels. Just before air could be pulled through eachfilter cake, the addition of 12 liters of water to the funnel was begunfor the purpose of washing the filter cake. Air was briefly pulledthrough the washed filter cake. The wet filter cake contained 14 percentsolids by weight. After being removed from the funnels, the wet filtercakes were stirred with a propeller type agitator to form a solid inliquid suspension. The suspension was dried in a Niro spray drier (inlettemperature about 360° C.; outlet temperature about 128° C.) to form abatch of dried reinforced precipitated silica. The product had a surfacearea of 204 m² /g, a pore diameter at the maximum of the volume poresize distribution function of 17.6 nm, and a total intruded volume of4.81 cm³ /g. The product was micronized in a fluid energy mill usingcompressed air as the working fluid.

Table 1 shows physical test methods used to characterize uncured andcured tread rubber compositions

                  TABLE 1                                                         ______________________________________                                        Physical Test Methods                                                         Rubber Property   Test Method                                                 ______________________________________                                        Cure              ASTM D 2084-92                                              Maximum Torque                                                                Minimum Torque                                                                TS.sub.2 Scorch                                                               T.sub.50 Cure                                                                 T.sub.90 Cure                                                                 Hardness          ASTM D 2240-86                                              Rebound           ISO 4662                                                    Tensile           ASTM D 412-87                                               Elongation to Break                                                           Break Strength                                                                Modulus                                                                       Molded Groove Tear Strength                                                                     ASTM D 2262-87, modified                                                      by excluding use of the                                                       fabric backing on the                                                         rubber specimen.                                            Damattia Cut Growth                                                                             ASTM D 813-87                                               Dynamic Properties                                                                              ASTM D 2231-87                                              Complex Modulus, G*                                                           Storage Modulus, G'                                                           Loss Modulus, G"                                                              ______________________________________                                    

Predictive tire performance characteristics of cured tread rubbercompositions can be made using laboratory data on cured rubber specimensif properly analyzed. Thus, using the Rheometrics RDAII dynamicmechanical spectrometer operated in the temperature sweep mode, data isgenerated that can be predictive of tire product performance whenanalyzed using the equations reported in Tire Science & Technology, 18,2-12 (1990), and summarized in Table 2, below.

                  TABLE 2                                                         ______________________________________                                        Predictive Tire Performance Equations                                                         Relevent                                                      Performance     Dynamic   Temperature,                                        Characteristic  Property  °C.                                          ______________________________________                                        Rolling Resistance                                                                            G"/(G*).sup.0.8                                                                         60                                                  Wet Traction    G"/(G*).sup.0.1                                                                          0                                                  Dry Traction    G"/(G*).sup.1.8                                                                          0                                                  Ice Traction    1/G*      -30                                                 Cornering Coefficient                                                                         G*        60                                                  ______________________________________                                    

EXAMPLES 1-4

Table 3 shows a formula of a rubber composition useful in lowfuel-consumption passenger tire treads.

                  TABLE 3                                                         ______________________________________                                        Silica-Filled Tread Composition, phr                                          ______________________________________                                        Styrene-Butadiene Rubber.sup.1                                                                         75                                                   Butadiene Rubber.sup.2   25                                                   Reinforcing Carbon Black Variable                                             Silica                   Variable                                             Supported Silane Coupling Agent.sup.3                                                                  Variable                                             Processing Oil.sup.4     25                                                   Antidegradants.sup.5      3.5                                                 Stearic Acid              2                                                   Sulfur                    1.4                                                 Zinc Oxide                2.5                                                 Accelerator.sup.6         3.7                                                 ______________________________________                                         .sup.1 Shell Cariflex ® 1215, Shell Chemical Co.                          .sup.2 BR 1207, Goodyear Tire & Rubber Co.                                    .sup.3 50% bis(3(triethoxysilyl)propyl) tetrasulfide; 50% N330 reinforcin     carbon black.                                                                 .sup.4 Sundex ® 8125, Sun Chemical Corp.                                  .sup.5 Wingstay ® 100 antioxidant, Goodyear Tire & Rubber Co., 2.0        phr; Sunolite ® 240, Witco Chemical Co., 1.5 phr.                         .sup.6 Santocure ® NS accelerator, Monsanto Chemical Co., 1.7 phr; DP     accelerator, Monsanto Chemical Co., 2.0 phr.                             

The ingredients in Table 3 were admixed according to ASTM D 3182-87 inthe order and in the amounts therein specified to form curable rubbercompositions. No coupling agent was used. Example 1 was made with 6.5phr of N-330 reinforcing carbon black and 65 phr of reinforcingreinforced amorphous precipitated silica made according to U.S. Pat. No.5,094,829 and having a BET nitrogen surface area of 223 m² /g and a porediameter at the maximum of the volume pore size distribution function of15.8 nm; Example 2 was made with 6.5 phr of N-330 reinforcing carbonblack and 65 phr of reinforcing reinforced amorphous precipitated silicamade according to U.S. Pat. No. 5,094,829 and having a BET nitrogensurface area of 240 m² /g and a pore diameter at the maximum of thevolume pore size distribution function of 14.7 nm. Control 1 was madewith 6.5 phr of N-330 reinforcing carbon black, 65 phr of N-110reinforcing carbon black, and no silica; Control 2 was made with 6.5 phrof N-330 reinforcing carbon black and 65 phr of amorphous precipitatedsilica having a BET nitrogen surface area of 150 m² /g and a porediameter at the maximum of the volume pore size distribution function of32.4 nm. Each of the curable compositions was formed into mold specimensand cured for 20 minutes at 150° C. Upon cooling, samples of the curedcompositions were tested for various physical properties based upon themethods shown in Table 1. The results of physical testing of the samplesare shown in Table 4. The results of dynamic analysis and the predictivetire performance equations shown in Table 2 are shown in Table 5.

                  TABLE 4                                                         ______________________________________                                        Relative Compound Physical Properties                                                      Examples   Controls                                              Rubber Property                                                                              1       2        1     2                                       ______________________________________                                        Torque, N · m                                                        Minimum        14.08   13.72    4.14  8.16                                    Maximum        32.60   31.44    25.50 27.53                                   TS.sub.2 Scorch, min                                                                         1.36    1.40     1.96  2.85                                    T.sub.40 Cure, min                                                                           3.50    3.74     3.29  6.21                                    T.sub.90 Cure, min                                                                           34.41   35.74    4.98  24.61                                   Hardness, Shore A                                                             @ 20° C.                                                                              75      75       72    66                                      @ 100° C.                                                                             80      77       73    70                                      Rebound, %                                                                    @ 23° C.                                                                              41.2    44.4     44.2  43.2                                    @ 100° C.                                                                             61.0    63.0     66.6  63.2                                    Break Strength, MPa                                                                          20.7    21.4     22.8  19.3                                    Elongation to Break, %                                                                       865     818      502   831                                     Modulus, MPa                                                                  @ 20%          1.27    1.06     1.05  0.86                                    @ 100%         1.66    1.53     2.69  1.27                                    @ 300%         4.31    4.42     12.78 3.70                                    Demattia Cut Growth,                                                                         12.1    8.5      Failed*                                                                             5.7                                     mm @ 36 kc                                                                    ______________________________________                                         *A sample fails when the cut growth is 24.0 mm prior to reaching 36 kc.  

                  TABLE 5                                                         ______________________________________                                        Results of Analysis of Predictive Tire Performance Equations                                 Examples  Controls                                             Predicted Tire Property                                                                        1      2        1    2                                       ______________________________________                                        Wet Traction, Dimensionless                                                                    5.74   5.08     6.84 4.57                                    Dry Traction, Dimensionless                                                                    0.77   0.73     0.95 0.89                                    Ice Traction, (MPa).sup.-1                                                                     1.49   1.26     1.02 1.53                                    Rolling Resistance,                                                                            2.02   2.18     3.58 2.25                                    Dimensionless                                                                 Cornering Coefficient, MPa                                                                     0.83   0.72     0.52 0.51                                    ______________________________________                                    

Results in Table 5 show that use of reinforcing reinforced amorphousprecipitated silica made according to U.S. Pat. No. 5,094,829 serves toincrease predicted tire ice traction, to increase predicted tirecornering coefficient, and to decrease predicted tire rolling resistancerelative to reinforcing carbon black, and serves to increase predictedtire wet traction and predicted tire cornering coefficient relative toan amorphous precipitated silica not made according to U.S. Pat. No.5,094,829.

The ingredients in Table 3 were admixed according to ASTM D 3182-7 inthe order and in the amounts therein specified to form curable rubbercompositions. Example 3 was made with 65 phr of reinforcing reinforcedamorphous precipitated silica made according to U.S. Pat. No. 5,094,829and having a BET nitrogen surface area of 223 m² /g and a pore diameterat the maximum of the volume pore size distribution function of 15.8 nm,and 13 phr of the supported silane coupling agent of Table 3; Example 4was made with 65 phr of reinforcing reinforced amorphous precipitatedsilica made according to U.S. Pat. No. 5,094,829 and having a BETnitrogen surface area of 240 m² /g and a pore diameter at the maximum ofthe volume pore size distribution function of 14.7 nm, and 13 phr of thesupported silane coupling agent of Table 3. The only carbon blackpresent in the compositions of Examples 3 and 4 was that introduced as aportion of the supported silane coupling agent. Control 1 was made with6.5 phr of N-330 reinforcing carbon black and 65 phr of N-110reinforcing carbon black, no silica, and no silane coupling agent aspreviously described; Control 3 was made with 6.5 phr of N-330reinforcing carbon black, 65 phr of amorphous precipitated silica havinga BET nitrogen surface area of 150 m² /g and a pore diameter at themaximum of the volume pore size distribution function of 32.4 nm, and 13phr of the supported silane coupling agent of Table 3. Each of thecurable compositions was formed into mold specimens and cured for 20minutes at 150° C. Upon cooling, samples of the cured compositions weretested for various physical properties based upon the methods shown inTable 1. The results of physical testing of the samples are shown inTable 6. The results of dynamic analysis and the predictive tireperformance equations shown in Table 2 are shown in Table 7.

                  TABLE 6                                                         ______________________________________                                        Relative Compound Physical Properties                                                      Examples   Controls                                              Rubber Property                                                                              3       4        1     3                                       ______________________________________                                        Torque, N · m                                                        Minimum        6.77    7.09     4.14  3.65                                    Maximum        29.74   28.85    25.50 32.80                                   TS.sub.2 Scorch, min                                                                         1.07    1.06     1.96  4.40                                    T.sub.50 Cure, min                                                                           7.43    7.24     3.29  8.80                                    T.sub.90 Cure, min                                                                           30.66   29.17    4.98  16.49                                   Hardness, Shore A                                                             @ 20° C.                                                                              72      70       72    72                                      @ 100° C.                                                                             75      75       73    73                                      Rebound, %                                                                    @ 23° C.                                                                              41.0    42.0     44.2  44.2                                    @ 100° C.                                                                             63.8    65.0     66.6  66.6                                    Break Strength, MPa                                                                          25.6    25.8     22.8  21.3                                    Elongation to Break, %                                                                       578     575      502   449                                     Modulus, MPa                                                                  @ 20%          1.15    1.11     1.05  1.09                                    @ 100%         2.30    2.28     2.69  2.91                                    @ 300%         9.05    9.31     12.78 12.65                                   Demattia Cut Growth,                                                                         16.4    18.7     Failed*                                                                             15.8                                    mm @ 36 kc                                                                    ______________________________________                                         *A sample fails when the cut growth is 24.0 mm prior to reaching 36 kc.  

                  TABLE 7                                                         ______________________________________                                        Results of Analysis of Predictive Tire Performance Equations                                 Examples  Controls                                             Predicted Tire Property                                                                        3      4        1    3                                       ______________________________________                                        Wet Traction, Dimensionless                                                                    4.80   4.02     6.84 3.74                                    Dry Traction, Dimensionless                                                                    0.96   1.20     0.95 1.03                                    Ice Traction, (MPa).sup.-1                                                                     1.34   1.42     1.02 1.26                                    Rolling Resistance,                                                                            2.13   1.97     3.58 2.08                                    Dimensionless                                                                 Cornering Coefficient, MPa                                                                     0.90   1.06     0.52 0.58                                    ______________________________________                                    

Results in Table 7 show that use of reinforcing reinforced amorphousprecipitated silica made according to U.S. Pat. No. 5,094,829 serves toincrease predicted tire ice traction and predicted tire corneringcoefficient.

Results in Tables 4 and 6 show that use of reinforcing reinforcedamorphous precipitated silica without coupling agent decreases cutgrowth which is considered to be a beneficial effect.

Results in Tables 5 and 7 show that use of reinforcing reinforcedamorphous precipitated silica without coupling agent increases predictedtire wet traction which is considered to be a beneficial effect.

EXAMPLES 5-6

Table 8 shows a formula of a rubber composition useful in lowfuel-consumption passenger tire treads.

                  TABLE 8                                                         ______________________________________                                        Reinforcing Carbon Black-Filled Tread Composition, phr                        ______________________________________                                        Styrene-Butadiene Rubber.sup.1                                                                        55                                                    Butadiene Rubber.sup.2  25                                                    Natural Rubber.sup.3    20                                                    Reinforcing Carbon Black                                                                              Variable                                              Silica                  Variable                                              Processing Oil.sup.4    25                                                    Antidegradants.sup.5     3.5                                                  Stearic Acid             2                                                    Sulfur                   1.4                                                  Zinc Oxide               2.5                                                  Accelerator.sup.6        3.7                                                  ______________________________________                                         .sup.1 Shell Cariflex ® 1215, Shell Chemical Co.                          .sup.2 BR 1207, Goodyear Tire & Rubber Co.                                    .sup.3 CV60.                                                                  .sup.4 Sundex ® 8125, Sun CHemical Corp.                                  .sup.5 Wingstay ® 100 antioxidant, Goodyear Tire & Rubber Co., 2.0        phr; Sunolite ® 240, Witco Chemical Co., 1.5 phr.                         .sup.6 Santocure ® NS accelerator, Monsanto Chemical Co., 1.7 phr; DP     accelerator, Monsanto Chemical Co., 2.0 phr.                             

The ingredients in Table 8 were admixed according to ASTM D 3182-87 inthe order and in the amounts therein specified to form curable rubbercompositions. No coupling agent was used. Example 5 was made with 65 phrof reinforcing reinforced amorphous precipitated silica made accordingto U.S. Pat. No. 5,094,829 and having a BET nitrogen surface area of 223m² /g and a pore diameter at the maximum of the volume pore sizedistribution function of 15.8 nm; Example 6 was made with 65 phr ofreinforcing reinforced amorphous precipitated silica made according toU.S. Pat. No. 5,094,829 and having a BET nitrogen surface area of 259 m²/g and a pore diameter at the maximum of the volume pore sizedistribution function of 12.0 nm. The compositions of Examples 5 and 6contained no carbon black. Control 4 was made with 65 phr of N-220reinforcing carbon black and no silica; Control 5 was made with 65 phrof amorphous precipitated silica having a BET nitrogen surface area of150 m² /g and a pore diameter at the maximum of the volume pore sizedistribution function of 32.4 nm. Each of the curable compositions wasformed into mold specimens and cured for 20 minutes at 150° C. Uponcooling, samples of the cured compositions were tested for variousphysical properties based upon the methods shown in Table 1. The resultsof physical testing of the samples are shown in Table 9. The results ofdynamic analysis and the predictive tire performance equations shown inTable 2 are shown in Table 10.

                  TABLE 9                                                         ______________________________________                                        Relative Compound Physical Properties                                                      Examples   Controls                                              Rubber Property                                                                              5       6        4     5                                       ______________________________________                                        Torque, N · m                                                        Minimum        1.6     1.6      2.7   1.4                                     Maximum        18.6    18.8     16.2  16.9                                    TS.sub.2 Scorch, min                                                                         1.6     1.6      1.2   1.4                                     T.sub.90 Cure, min                                                                           3.7     3.8      2.7   3.3                                     Break Strength, MPa                                                                          22.0    20.8     21.3  19.9                                    Elongation to Break, %                                                                       742     707      604   689                                     Modulus, MPa                                                                  @ 20%          0.87    0.90     0.86  0.82                                    @ 300%         6.1     6.7      8.7   6.1                                     Molded Groove Tear                                                                           17.9    16.1     12.1  14.0                                    Strength, kN/m                                                                Demattia Cut Growth,                                                                         12.6    12.0     22.6  15.0                                    mm @ 36 kc                                                                    ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        Results of Analysis of Predictive Tire Performance Equations                                 Examples  Controls                                             Predicted Tire Property                                                                        5      6        4    5                                       ______________________________________                                        Wet Traction, Dimensionless                                                                    4.34   3.88     4.85 4.37                                    Dry Traction, Dimensionless                                                                    1.45   1.61     1.10 1.18                                    Ice Traction, (MPa).sup.-1                                                                     1.80   1.61     1.68 1.68                                    Rolling Resistance,                                                                            3.10   2.88     3.49 3.32                                    Dimensionless                                                                 Cornering Coefficient, MPa                                                                     0.84   0.90     0.72 0.71                                    ______________________________________                                    

Results in Table 10 show that use of reinforcing reinforced amorphousprecipitated silica made according to U.S. Pat. No. 5,094,829 serves toincrease predicted tire dry traction and predicted tire corneringcoefficient and to decrease predicted tire rolling resistance relativeto reinforcing carbon black and serves to increase predicted tire drytraction and predicted tire cornering coefficient relative to a silicanot made according to U.S. Pat. No. 5,094,829.

Although the present invention has been described with reference tospecific details of certain embodiments thereof, it is not intended thatsuch details should be regarded as limitations upon the scope of theinvention except insofar as they are included in the accompanyingclaims.

We claim:
 1. In a tire comprising:(a) a carcass having a crown; and (b) cured tread rubber composition adhered to said crown of said carcass;the improvement wherein said cured tread rubber composition comprises in combination: (c) organic rubber; (d) from 0 to 20 phr of reinforcing carbon black; and (e) from 40 to 120 phr of reinforcing reinforced amorphous precipitated silica wherein said reinforcing reinforced amorphous precipitated silica has a surface area of from 160 to 340 m² /g and a pore diameter at the maximum of the volume pore size distribution function of from 5 to 19 nm.
 2. The tire of claim 1 wherein said organic rubber constitutes from 20 to 70 percent by weight of said cured tread rubber composition.
 3. The tire of claim 1 wherein:(a) said organic rubber constitutes from 30 to 65 percent by weight of said cured tread rubber composition; (b) said reinforcing carbon black constitutes from 0 to 15 phr of said cured tread rubber composition; and (c) said reinforcing reinforced amorphous precipitated silica constitutes from 40 to 100 phr of said cured tread rubber composition.
 4. The tire of claim 1 wherein:(a) said organic rubber constitutes from 37 to 60 percent by weight of said cured tread rubber composition; (b) said reinforcing carbon black constitutes from 0 to 10 phr of said cured tread rubber composition; and (c) said reinforcing reinforced amorphous precipitated silica constitutes from 40 to 80 phr of said cured tread rubber composition.
 5. The tire of claim 1 wherein said reinforcing reinforced amorphous precipitated silica has a surface area of from 180 to 340 m² /g.
 6. The tire of claim 1 wherein said reinforcing reinforced amorphous precipitated silica has a surface area of from 200 to 340 m² /g.
 7. The tire of claim 1 wherein said reinforcing reinforced amorphous precipitated silica has a pore diameter at the maximum of the volume pore size distribution function of from 8 to 18 nm.
 8. The tire of claim 1 wherein said reinforcing reinforced amorphous precipitated silica has an average ultimate particle size of less than 0.1 nm.
 9. The tire of claim 1 wherein said organic rubber comprises natural rubber, cis-1,4-polyisoprene, cis-1,4-polybutadiene, trans-1,4-polybutadiene, 1,2-polybutadiene, co-(styrene-butadiene), acrylonitrile-based rubber composition, isobutylene-based rubber composition or a mixture thereof.
 10. The tire of claim 1 which is substantially free of silane coupling agent.
 11. The tire of claim 1 which is substantially free of coupling agent.
 12. The tire of claim 11 wherein said organic rubber constitutes from 20 to 70 percent by weight of said cured tread rubber composition.
 13. The tire of claim 11 wherein:(a) said organic rubber constitutes from 30 to 65 percent by weight of said cured tread rubber composition; (b) said reinforcing carbon black constitutes from 0 to 15 phr of said cured tread rubber composition; and (c) said reinforcing reinforced amorphous precipitated silica constitutes from 40 to 100 phr of said cured tread rubber composition.
 14. The tire of claim 11 wherein:(a) said organic rubber constitutes from 37 to 60 percent by weight of said cured tread rubber composition; (b) said reinforcing carbon black constitutes from 0 to 10 phr of said cured tread rubber composition; and (c) said reinforcing reinforced amorphous precipitated silica constitutes from 40 to 80 phr of said cured tread rubber composition.
 15. The tire of claim 11 wherein said reinforcing reinforced amorphous precipitated silica has a surface area of from 180 to 340 m² /g.
 16. The tire of claim 11 wherein said reinforcing reinforced amorphous precipitated silica has a surface area of from 200 to 340 m² /g.
 17. The tire of claim 11 wherein said reinforcing reinforced amorphous precipitated silica has a pore diameter at the maximum of the volume pore size distribution function of from 8 to 18 nm.
 18. The tire of claim 11 wherein said reinforcing reinforced amorphous precipitated silica has an average ultimate particle size of less than 0.1 nm.
 19. The tire of claim 11 wherein said organic rubber comprises natural rubber, cis-1,4-polyisoprene, cis-1,4-polybutadiene, trans-1,4-polybutadiene, 1,2-polybutadiene, co-(styrene-butadiene), acrylonitrile-based rubber composition, isobutylene-based rubber composition or a mixture thereof. 